The Global Positioning System (GPS) enables determination of a position of a user with a GPS receiver in time and space; that is, the x, y, z and t coordinates of the GPS receiver. In its simplest term, this is accomplished by the triangulation between a number of orbiting satellites with known geo-locations. The range between a GPS satellite and the GPS receiver is obtained by multiplying the radio signal transmission time by the speed of light. This range is often called "pseudorange", however, since various errors including timing errors make this range different from the real distance between the GPS satellite and the GPS receiver. At least four GPS orbiting satellites are needed to uniquely determine a position plus the clock for a user. Additional satellites are desirable for further correcting timing errors and other errors and using redundancy for cross checking the measurements.
Time stamps are received as positioning from each of the satellites, to inform the position of the satellite and the time of the transmission. The user can triangulate using this information to determine the position at which the information was received, with high accuracy.
The accuracy of this positioning detection, however, is intentionally limited. A U.S. policy directive limits the amount of accuracy that is obtainable from GPS. A dither algorithm, called Selective Availability ("SA") and used as a part of the signals transmitted by the GPS, ensures that the clock and ranging data are only accurate to the level of 50-100 meters without knowledge of the dithering algorithm. Accordingly, only those authorized users who are permitted by the Department of Defense to obtain the dither key can receive clock and ranging data to a few-meter accuracy. The GIPSY-OASIS II software provides techniques to substantially improve positioning accuracy to the sub-centimeter level both for civilian users of GPS who are affected by the SA dither and for the users authorized for knowledge of the dither key.
Various techniques have been used in the prior art to improve the resolution of the data available from the Global Positioning System (GPS). These techniques are well-known in the art. Jet Propulsion Laboratory (JPL), the assignee of this application has been very active in this area since 1979. The results of the continuous effort at JPL for over a decade are a versatile processing system, GIPSY-OASIS II. This software was initially developed at JPL to support various precise differential GPS applications. GIPSY-OASIS II includes two major elements: the GPS-Inferred Positioning System (GIPSY)for processing actual GPS data; and the Orbit Analysis and Simulation Software (OASIS) for performing simulations, covariance analyses, and system design trade studies.
The principal applications currently supported by GIPSY-OASIS II include precise orbit determination for low earth satellites that carry GPS receivers; daily determination of GPS satellite precise orbits and relative clock offsets for the International GPS Service; and numerous investigations in regional and global geodesy for the study of solid earth dynamics. Each of these applications employs data collected by a permanent network of GPS reference receivers, which now number more than 150 sites worldwide, and each involves the continuous computation of GPS satellite orbits and clock offsets as well as all ground receiver clock offsets from a network reference time. For the past two years, the 3D accuracy for the GPS satellite orbits computed with GIPSY-OASIS has been 10-15 cm (RMS), and relative clock offset accuracies between ground sites have better than a few tenths of one nanosecond worldwide. The estimated GPS orbits can be propagated forward for about 1-2 days with a 3D RMS accuracy of 1 m or better, as demonstrated by direct comparison of predicted and filtered orbits.
The commercially-available GIPSY-OASIS II system has been used for GPS data analysis, automated GPS orbit production, many differential GPS applications, and in particular, many non-GPS analysis capabilities. JPL currently-collects GPS data every day from a global network of receivers and each day estimates all GPS orbits, all receiver locations, all clocks, and a host of geodetic and miscellaneous parameters. The daily repeatability of the 3D global geocentric station locations is better than 1 cm, far surpassing the 50-100 meters accuracies typical of ordinary users or the few meters of accuracy typical of authorized users. The GIPSY-OASIS II architecture offers unique advantages such as high accuracy at centimeter level for both ground and space applications, automated and fast operations, and adaptability to non-GPS applications. Therefore, this software has attracted numerous applications and users worldwide including applications in GPS, non-GPS orbiters, FAA, military and users in commercial sectors, government agencies, universities and research institutions.
GIPSY-OASIS II processes the data through a general Kalman sequential filter smoother which runs considerably faster than real time for its current tasks on various standard UNIX workstations. For many applications the system is configured to operate entirely automatically and can deliver validated daily solutions for orbits, clocks, station locations, and many other parameters, conveniently with no human intervention.
GPS phase and pseudorange data are retrieved automatically (generally through the Internet or phone lines) from each station, passed on to an expert system which edits and validates them, and then delivers them to the estimation sequence. Extensive post-estimation verification is carried out so that anomalous points, if any, are deleted and the solution readjusted through a fast down-dating process (rather like a Kalman update done in reverse), and detailed performance statistics are generated, again automatically.
GIPSY-OASIS II has been developed as a general purpose GPS data analysis system. The system is highly modular to allow a maximal flexibility. There are about dozen key modules and sub-modules that may be called in sequence (or a sub-sequence may be iterated) in the course of an estimation process. Different modules are independent to each other but their operations are controlled by a higher-level executive having a plurality of UNIX shell scripts. In a routine or automated operation, the UNIX shell scripts effectively take the place of an analyst to control and monitor the data analysis and simulation. In a non-standard application, an analyst may choose to execute a group of particular modules individually. In addition, the UNIX shell scripts in GIPSY-OASIS II can be complex and sophisticated to minimize processing faults and to perform everything from intelligent memory and file management to exhaustive verification and correction of the computed products.
One virtue of this architecture of independent subordinate modules and controlling UNIX shell scripts is that GIPSY-OASIS II can be applied to virtually any GPS estimation problem, optimized and automated for that purpose, simply by modifying (or creating) a relatively small amount of UNIX script. In most cases the GIPSY-OASIS code itself (e.g., editing and data conditioning, model and partial derivative computation, filtering and smoothing, and post fit validation), which has been optimized for numerical stability, precision, and computational efficiency over many years of demanding use, need not be touched at all.
It is a first objective of the present invention to even further refine this accuracy, however, using some new models which have been found to relieve some of the limitations of the previously released GIPSY-OASIS II system.
It is another object of the present invention to improve the accuracy available from the GPS in a new and unobvious way.
It is yet another object of the present invention to define special techniques which can be used to manipulate global positioning satellite information in a way which enhances the usability of this information.
It is still another object of the present invention to adapt these techniques to other positioning tools besides the GPS system.
In accordance with the invention, one preferred method for GPS positioning and satellite tracking comprises: collecting broadcast signals of a plurality of earth-orbiting satellites with a plurality of earth-fixed receivers, said broadcast signals being indicative of timing and positioning of said satellites; converting said broadcast signals into raw data in digital form; editing said raw data to remove an amount of data points based on a data selection criterion and to detect carrier phase breaks therein based on a phase criterion, thus producing refined data indicative of pseudoranges of said satellites; computing a plurality of forces acting on said satellites; estimating effects of earth geometric factors and a plurality of transmitting delays of said broadcast signals; performing a yaw compensation on yawing of said satellites; computing orbiting trajectories of said satellites by using a priori model and said refined data with information indicative of said forces, said effects of earth geometric factors, said transmitting delays, and said yawing; producing updated data of said satellites by using a Kalman-type filter/smoother and said orbiting trajectories of said satellites, said updated data having information indicative of said timing and said positioning of satellites; predicting said orbiting of said satellites with said updated data and information from said yaw compensation to produce updated orbits of said satellites; and verifying said updated orbits of said satellites.
Additional objects and advantages of the present invention will be set forth in part in the detailed description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.